The present invention relates generally to automatic focusing systems, and is applicable particularly to video cameras having a zooming function.
Various types of video cameras are known which are generally provided with an optical system comprising a focus lens and a zoom lens so that focusing operation is effected by movement of the focus lens in the directions along the optical axis of the optical system and further zooming operation is performed by movement of the zoom lens in the same optical axis directions.
In such a video camera, when the position of the zoom lens on the optical axis is shifted from the telephoto side to the wide-angle side or vice versa, the focus lens is moved in accordance with the distance to an object to be taken in connection with the movement of the zoom lens so as to always keep the focusing state.
In the above-mentioned video camera having a zooming function, the relation between the positions of the focus lens and the zoom lens on the optical axis, as shown in FIG. 7, varies in accordance with distances u.sub.1, u.sub.2, . . . , u.sub.n to the object to be taken in focus by the video camera. As obvious from curves showing the relation between the position of the focus lens and the position of the zoom lens, as the distance to the object is shorter, the inclination is smaller as indicated by the curve u.sub.n, whereas in accordance with lengthening of the distance to the object, the inclination is greater as indicated by the curve u.sub.1. Accordingly, in cases where the distance to the object is considerably short, there is a problem which arises with the prior art video cameras in that, when the zoom lens is positioned at the telephoto side, difficulty is encountered to attain the focusing state in the movable range of the focus lens.